1. Field of the Invention
The invention relates to a device for transporting a recording medium in a printer or photocopier and to the use of a device of this type in a fuser station of a printer or photocopier.
2. Description of the Related Art
It is customary to use endless paper with transportation perforations in high speed non-mechanical printing apparatuses which operate by the electrophotographic or magnetic principle. In this case, the endless paper is withdrawn from a stack, transported through a transfer station with the aid of tractors engaging in the edge perforations and thereby coated with a layer of toner depending on the characters being printed. The layer of toner situated on the recording medium is then thermally fused in a fuser station, also termed a fixing station. A fuser station of this type is known, for example, from U.S. Pat. No. 4,147,922. For this purpose, the fuser station comprises in a customary manner an electric, heated fuser, or fixing roller and a feed roller which can be pivoted onto and away from the fuser roller. In this case, the recording medium must be guided precisely through the fuser station without movement relative to the fuser roller.
In order to prevent relative movement of this type, it has proved necessary to ensure an even pressing force of the feed roller onto the fuser roller along the length of the roller. With different pressing forces along the length of the roller, there is a risk of the web of paper migrating horizontally in the fuser station during the printing operation. This can lead to tensioning of the web of paper and, with high transport speeds, to a tear in the paper.
For the pivoting of a feed roller onto and away from a counter-roller in a fuser station, it is known from German Patent 27 17 260 to arrange the feed roller on links and to pivot the latter via cam disks. In this case, the counter-roller is mounted in a stationary manner.
If the feed roller is not guided and pivoted on precisely parallel to the counter-roller, different pressing forces act along the rollers. Although the elastic sheathing of the feed roller ensures a certain force compensation, it has been shown that this force compensation is not sufficient.
A fusing apparatus for photocopiers is known from European Published Application 0,002,303, which has a fuser roller and a feed roller which can be pivoted onto and away from the fuser roller. The feed roller is mounted so as to be pivotable in a support cage, on which an eccentric member engages. The pressing force of the feed roller onto the fuser roller is measured via an apparatus measuring the deflection of the support cage. By pivoting the eccentric, the pressing force of the feed roller along the fuser roller can be changed or adjusted. Pressing fluctuations of the feed roller onto the fuser roller can thus be compensated.
Furthermore, the recording media used can vary in thickness over their width, or the recording media serve as media for credit cards to be printed, etc.
Additionally, non-mechanical printing apparatuses can be designed in such a way that they can process webs of recording media of different widths. With these, it is necessary to design the fuser station with the rollers arranged therein in accordance with the greatest width of recording medium. Regions of the fuser roller or of the feed roller in the fuser station which do not come into contact with the recording medium heat up locally more intensely than the actual contact regions with the recording medium because the recording medium withdraws heat from the fuser roller in the fusing region during the fusing operation. This leads to a heat-dependent different diameter of the feed roller consisting of elastic material. As a result, the recording medium is deflected horizontally.
It has therefore proved favorable to monitor the lateral position of the recording medium precisely via a scanning apparatus when it enters the fuser station in order to be able to intervene, if necessary, in terms of control via a control arrangement.
However, the problem of the precise controllable guiding of recording media is not only present wish electrophotographic printers and photocopiers. Even with other printing apparatuses which operate, for example, with inking units, it is necessary to position the recording medium precisely, at least in the printing region.
For positioning the recording media in printing apparatuses, it is customary to use electrically adjustable mechanical guide elements in the paper duct of the printing apparatus, the scanning of the ACTUAL position of the recording media likewise taking place via mechanical contact elements.
For scanning the position of the recording media, it is known from the IBM Technical Disclosure Bulletin, Vol. 23, No. 7a, December 1980 and from European Patent 0 031 137 to use optoelectronic scanning apparatuses.